1. Field of the Invention
The present invention generally relates to quickly accessing a portion of data stored on a sequentially accessible storage device. More particularly, the invention concerns a method and apparatus which retains on a direct-access storage device (DASD) a portion of the digital data transferred to a sequential-access storage media, and facilitates fast access to that portion of digital data. The format of the portion of digital data retained in the DASD generally emulates the format of the sequential-access storage media.
2. Description of the Related Art
Over the years, engineers have developed many different ways of digitally storing data. Two of the most popular involve sequential-access storage drives (i.e., tape media), and DASD. As an example, DASD storage includes magnetic hard disk drives and magnetic floppy diskettes. Tape storage, for instance, includes electromagnetic or optical tapes, and may involve storage patterns such as linear, helical, and serpentine patterns.
DASD and tape storage offer contrasting benefits and limitations. Thus, users may choose DASD for some storage needs, and tape for others. As would be expected, DASD and tape storage have each evolved through the years with contrasting storage formats. This is due to the different physical characteristics of the respective media. In particular, DASD--stored data is randomly accessible, by simply moving a read/write head directly to a specific sector and track location where a desired data item exists. In contrast, tape-stored data is accessed sequentially, by rewinding or advancing the tape until reaching the desired data item.
In tape storage, data is typically stored in units called "records" or "blocks". Interspersed within the records are various headers, each of which contains information describing various characteristics of the associated data record. Adjacent header/record pairs may be separated by an inter-block gap (IBG), which is a unique hardware-recognizable pattern of stored data. The IBGs are particularly advantageous when attempting to locate a particular data record on the tape. First, the tape is advanced to a point on the tape where the data record likely begins, or to a point where a group of records including the data record begins. Then, the tape head advances or rewinds, record-by-record, until the desired record is reached. Such movement of the tape head is conducted using the IBGs as guideposts.
It is common practice for the first few records written on the tape to contain information about the tape volume and the data recorded on it. The collection of records containing the information is typically called the volume label. In the mainframe operating system type environment ("MVS"), the information contained in the volume label is used by a tape management system to determine who owns a tape volume and if the data contained on the volume has expired and can be overwritten by an application program. This means that prior to writing new data from the beginning of a tape volume, at least the first set of records containing the volume label must be read by the host operating system.
In addition to individual record or "block" headers, a tape usually includes a volume header near the beginning of tape (BOT) point. The volume header includes various statistics concerning all records contained on the tape. The volume header is necessarily placed at the tape's beginning because, when a tape cartridge is first loaded to a tape drive, access of the tape begins at the BOT point. To locate the volume header elsewhere would require time consuming forwarding of the tape to reach the volume header.
The foregoing description of tape storage significantly contrasts with DASD storage. Although most formats of DASD storage use headers of various types, DASD storage does not require IBGs. This is because the DASD read/write head, by virtue of its inherent random access capability, can proceed directly to the desired data, without having to sequence record-by-record to find a desired record.
With the advent of the virtual tape system (VTS), the line between DASD and tape storage has blurred. A VTS stores data on tape, taking advantage of this inexpensive means of long term data storage. However, to expedite data exchanges, data is cached in a DASD. According to a predetermined criteria, such as the data's age or recency/frequency of use, the data is backed up on tape. Cache misses may result in older data being retrieved from tape and stored again in the DASD cache.
To maintain a consistent data storage format in VTS, it is desirable to use one substantially common tape-like storage format for both DASD and tape media. When this tape-like storage format is applied to DASD, however, some of the normal mechanisms for locating data on tape are no longer useful. Chiefly, the data is not stored with IBGs, which are inapplicable to the direct-access storage format. Even with IBGs, however, access to DASD data would be severely retarded by requiring the DASD head to sequence record-by-record through the data, when direct access is possible. Thus, certain improvements to strict tape storage formats as stored on the DASD are needed as an improved process for accessing the data to take advantage of the inherently rapid data access potential of DASD storage.
Furthermore, improvements in the methods used to store and access data cached in a DASD used in a VTS are also desirable. Current methods, although faster than methods requiring a complete volume of data to be read from tape storage, need to be improved to allow faster access to at least portions of the data stored on tape.
As described previously, some operating systems depend on reading the first few data records on the tape prior to allowing new data to be written from the beginning of the tape. In prior art VTS systems, the older data for a volume must be retrieved from tape and stored again in the DASD cache, even though only the first few records are utilized. The present invention eliminates the need to retrieve the older data and allows for faster access to the data.